There is a great deal of discussion about the conditions under which monarchs should be tagged in the western states. The population is indeed much smaller and may total only a few hundred thousand monarchs in a bad year. In the eastern portion of the US it is possible to tag 100,000 monarchs and yet the ratio of tagged to untagged monarchs is still 1/20,000 (only .00005 0f the total population). There is a good chance that if 30,000 monarchs were tagged in the west that this number might represent 15-30% of the population. When one is dealing with a smaller population, several concerns arise. Are there handling effects associated with the tagging that contribute to the mortality of the butterflies, and if so, even if such effects are slight, is there a way to minimize the impact of tagging. Also, since one is dealing with a small population (of both monarchs and people) and a large territory west of the Rockies, are recapture rates high enough to justify the tagging effort. In the eastern migration, recapture rates run from about a half a percent to 5%. These are good rates but it is possible that proportions of recaptures in the west, especially of monarchs tagged at overwintering sites and dispersing in the spring could be much less. The bottom line is that the objectives of the tagging have to be clearly drawn. Should tagging be encouraged for monarchs originating in western states with the hope that the recoveries at overwintering sites along the California coast will reveal more about where monarchs come from that colonize these coastal locations? Should monarchs only be tagged at one or a few of the overwintering sites to determine how much they move from site to site or on and off sites through the winter months? Should the tagging be conducted in order to get a better estimate of the size of the population? Should the tagging and measuring be conducted to get a sense of the changes in individual butterflies through the season (by measurement at each recapture)?
Because scientists don’t know whether the monarchs east and west of the Rockies are genetically different, every effort is being made not to move monarchs from one side of the Rockies to the other. However, both populations are thought to represent the same species.